fix: Update Span's == argument types to Self

Resolves a compilation failure with recent Swift nightly toolchains...
`error: member operator '==' of protocol 'Span' must have at least one argument of type 'Self'`

Author: clackary

Sources/OpenTelemetryApi/Trace/Span.swift

@@ -74,7 +74,7 @@ public extension Span {
         hasher.combine(context.spanId)
     }
 
-    static func == (lhs: Span, rhs: Span) -> Bool {
+    static func == (lhs: Self, rhs: Self) -> Bool {
         return lhs.context.spanId == rhs.context.spanId
     }
 }